The power output from a gas turbine engine is directly proportional to both the temperature at which it operates and the quantity of air that flows through it. For a conventional gas turbine engine in which each compressor is powered by an independent turbine, the relationship between core flow and temperatures is fixed. In order to increase the power output from the engine more fuel must be burned in the combustor. This increases both core flow and temperatures. The operating temperatures of the engine must be closely monitored and the fuel supply regulated to prevent the temperature capability of the engine being exceeded. For an engine of fixed physical size and pressure ratio (and hence core flow) the temperature capability of the engine therefore places a practical limit on the power output available from it.
This is illustrated in FIG. 1 which shows a schematic graph of engine operating temperature against core flow, and plots as a thick solid line the relationship between temperature and core flow of a given conventional engine. As indicated by the arrowed line, changing the fuel flow changes the position of the engine on the line. Also plotted on the graph are contours of equal power output. Combining these with the temperature capability limit of the engine shows that there is a maximum power output that can be obtained from the engine, determined by that limit.
If the maximum available power is less than that required, then either the temperature capability of the engine must be increased (e.g. by using higher grade materials or increasing cooling flows within the engine hot section), or the engine must be redesigned to increase the amount of core flow that passes through the engine. This can be achieved by increasing the physical size of the engine or increasing the core pressure ratio.
The fixed core flow vs. temperature relationship of a conventional gas turbine has two major implications for its design and operation. Firstly, the engine must be physically sized and the temperature capability of the components designed for the most arduous operating conditions (i.e. maximum power output requirement). For a civil aero engine this is typically take-off operation on a warm day. Much of the time the engine will operate at less arduous conditions (e.g. cruise) where the reduced power demand could be supplied more efficiently by a smaller engine or one of reduced temperature capability.
Secondly, as the engine deteriorates over time, its operating temperatures increase due to the reduction in component efficiencies. The maximum power output capability of the engine therefore reduces until it is no longer able to supply the required power. A new engine must therefore be over-designed to accommodate the effects of deterioration.
The compressors of conventional gas turbine engines typically have variable stator vanes (VSVs) which are used to increase the compressor stability margin at off design conditions by controlling the incidence onto the blading and the distribution of work between the stages of the compressor. As the VSVs are rotated the spool speed changes, such that the work done by the compressor remains approximately constant. The core flow vs. temperature relationship of the engine therefore remains unchanged when the VSVs are adjusted. Typically the VSVs are scheduled to progressively open as the engine speed increases. By “variable stator vanes” we mean both vanes that are located between rows of rotating compressor blades and vanes (often called “inlet guide vanes”) which are located at the inlet to the compressor.